1. Field of the Invention
The present invention relates to a composite crucible used for manufacturing a silicon crystal, and a method of manufacturing the crucible.
2. Description of the Related Art
Nowadays, the demand for a solar cell has been increasing in consideration of environment problem and energy problem. The solar cell is generally classified into a silicon solar cell and a compound semiconductor solar cell, based on the kind of the semiconductor used for the power generation part. Furthermore, the silicon solar cell is classified into a crystalline silicon solar cell and an amorphous silicon solar cell. The crystalline silicon solar cell is classified into a monocrystalline silicon solar cell and a polycrystalline silicon solar cell.
Regarding conversion efficiency which is the most important aspect for a solar cell, nowadays, the compound semiconductor solar cell has reached nearly 25%, which is the highest, and the monocrystalline silicon solar cell is about 20%, and the polycrystalline silicon solar cell and the amorphous silicon solar cell are 5 to 15%. Regarding the raw material cost, silicon is an element which is the second most abundant on the earth (oxygen is the most abundant), and thus much cheaper than compound semiconductor, and thus silicon solar cell is most widely used. The conversion efficiency refers to a ratio of the energy converted to electrical energy by the solar cell with respect to the energy of the incoming light into the solar cell. The ratio is represented by “percentage (%)”.
Next, a method of manufacturing a monocrystalline silicon solar cell will be explained briefly. First, a cylindrical silicon single crystal ingot is manufactured by the Czochralski method (the CZ method) or the floating zone melting method (the FZ method). For example, in the CZ method, polycrystalline silicon is supplied into a vitreous silica crucible, followed by melting by heating. Then, a seed crystal is dipped into the obtained silicon melt, and gradually pulled up, to produce a silicon single crystal.
Then, the ingot is sliced to obtain thin wafers each having a thickness of, for example, 300 μm. The surface of the obtained wafer is etched by a chemical to remove processing strain in the surface, to obtain a wafer (substrate) for a solar cell. An Impurity (dopant) is diffused into the wafer to form a PN junction on one side of the wafer, and then electrodes are formed on both sides, and thereafter an antireflection film is formed on the sunlight incidence plane to reduce light energy loss due to light reflection, to obtain the solar cell. In the solar cell, in order to increase the current, it is important to manufacture a solar cell having a larger area. The CZ method is excellent in that it allows easy production of a silicon single crystal having a large diameter, and the obtained single crystal is excellent in the strength. Therefore, this method is preferred as a method of obtaining a silicon wafer, having a large diameter, which is a substrate material for manufacturing a large-area solar cell.
On the other hand, in the manufacturing of the polycrystalline silicon solar cell, it is preferred to use the casting method where silicon melt is solidified in the mold, or the electromagnetic casting method which is a continuous casting method by electromagnetic induction. By use of such method, it is possible to manufacture a substrate material at lower cost than single crystal silicon substrate, which is manufactured by the CZ method. In the casting method, high-purity silicon is heated and melted in a crucible, and a small amount of boron (which is a dopant) and the like is uniformly added thereto, and the silicon melt is solidified therein or solidified after the silicon melt is introduced into a mold. It is required that the crucible and the mold used for the casting method are excellent in heat resistance and shape stability, and in addition low impurity content is also demanded. So, the crucible is made of silica, and the mold is made of graphite.
The vitreous silica crucible used for manufacturing a silicon crystal is required to have high viscosity at high temperature in order to be durable enough for long-time and multiple pulling or casting. Furthermore, the vitreous silica crucible is required to be manufactured at low cost. As a conventional crucible having high strength at high temperature, there are known a crucible having a layer containing aluminium (Al) in high concentration at the outer surface side, a crucible obtained by coating a crystallization promoter (e.g. barium (Ba)) on the outer surface, and a crucible having, on the outer surface, a stabilization layer made of alumina, mullite, or the like (See JP-A-2000-247778, JP-A-2008-507467, and JP-A-2004-531449). Another related reference is JP-A-H1-153579.